This invention relates to the active electronic cancellation of vibration emitted from operating D.C. motors, especially motor related tonal vibration. The use of active noise control in D.C. motors allows for savings in space and weight by replacement of selected conventional noise and vibration absorption methods with active control. Critical ways to eliminate this noise are by canceling the drive motor slot noise or other motor related tonals such as SCR vibration.
DC motors come in various configurations but a typical configuration is a separately excited, DC motor where three phase 60 Hz power from an AC source E such as a diesel motor generator is full wave rectified with six SCR's. per drive motor. The phase angle relative to line frequency during which the SCR's. turn on is varied to control the speed of the unit to be driven.
Tonal noise appears at frequencies related to the passage of armature slots within the motors or to harmonics of line frequency if an SCR drive is used. If multiple motors are run at different speeds then slot tones appear, one for each of the motor rates.
Noise Cancellation Technologies, Inc. (NCT) has devised controllers which, when coupled to the circuit described herein, can actively control the slot noise. NCT has developed several adaptive active cancellation controllers such as the NCT 2000 & 2010 both commercially available that have been used successfully to cancel periodic noise which arises, typically, from rotating machinery or repetitive sources. These controllers eliminate noise by adapting the coefficients of cosine and sine components of the frequencies to be canceled to produce an 180.degree. C. out of phase signal to cancel the tonal noise. This results in very selective cancellation of the tonals related to the actual rate at which the equipment is operating. Adaptation to changes in the noise is very rapid. Also, the need for a reference signal of the actual noise from each source is eliminated and only synchronizing speed signals from the sources are needed. As the cancellation process is adaptive, it is not necessary to know the exact time delay and multipath structure between the device being quieted and the individual sensor elements. The algorithm will adapt in changes in phase and amplitude of the noise signal as long as the rate of change of the signal structure does not occur faster than the algorithm can adapt. An example of the fast adapting algorithm of NCT to variations in thee amplitude and phase of the noise is shown and described in U.S. Pat. No. 4,878,188 which is incorporated herein by reference. Adaption signal is controlled by a parameter that determines the bandwidth of the canceling signal. Increasing the speed of adaption increases the bandwidth of the canceling signal. Selection of the parameter value is dependent upon finding an optimum trade off between the need to track time varying propagation characteristics and the desire to minimize the bandwidth of the canceling signal.
This invention also makes use of the technique of using multiple sensors and actuators to generate the optimum control signal detailed in U.S. Pat. No. 5,091,953 "Repetitive Phenomena Cancellation Arrangement with Multiple Sensors and Actuators" which is hereby incorporated herein by reference. This technique can make use of multiple input signals and can generate multiple output signals that do not interfere with each other. That is, the controller will not try to cancel a signal from a actuator. It subtracts out these actuator signals from the control error minimization function by obtaining transfer functions between all sensors and actuators during calibration. Therefore, the algorithm knows what any particular actuator signal will look like at any sensor and subtracts it out of each sensor signal.
In the DC motor application, one or more accelerometers are used as the sensors and the field and/or armature currents are used as the extractors. During calibration, the transfer functions between field excitation and the sensors and between armature excitation and sensors are determined. Upon enabling the controller, the sensor signals are minimized at the frequencies of interest. Note that this algorithm is robust in that disconnecting one or more sensors and/or one actuator will not cause the system to become unstable; instead, the algorithm simples calculates new control signal(s) based on the remaining sensor information available to it.
In this invention, slot noise results as torque impulses are induced from the rotor to the stator of the drive motors as the armature rotates. These torque impulses are induced from the rotor to the stator of the drive motors as the armature rotates. These torque impulses are induced as slots on the armature pass by the motor stator poles. Use of helical slots reduces but does not eliminate the torque impulses.
Generally, the electrical power to the drive motor is modulated thus producing counter forces within the motor itself which act to counter the slot related impulses. To provide a feedback signal, transducers, such as accelerometers, which can sense slot rate vibration are mounted either on the motor or its mounting bracket/foundation to provide a feedback signal to the active cancellation controller.